2,718 research outputs found
Dust photophoretic transport around a T Tauri star: Implications for comets composition
There is a growing body of evidences for the presence of crystalline material
in comets. These crystals are believed to have been annealed in the inner part
of the proto-solar nebula, while comets should have been formed in the outer
regions. Several transport processes have been proposed to reconcile these two
facts; among them a migration driven by photophoresis. The primarily goal of
this work is to assess whether disk irradiation by a Pre-Main Sequence star
would influence the photophoretic transport. To do so, we have implemented an
evolving 1+1D model of an accretion disk, including advanced numerical
techniques, undergoing a time-dependent irradiation, consistent with the
evolution of the proto-Sun along the Pre-Main Sequence. The photophoresis is
described using a formalism introduced in several previous works. Adopting the
opacity prescription used in these former studies, we find that the disk
irradiation enhances the photophoretic transport: the assumption of a disk
central hole of several astronomical units in radius is no longer strictly
required, whereas the need for an ad hoc introduction of photoevaporation is
reduced. However, we show that a residual trail of small particles could
annihilate the photophoretic driven transport via their effect on the opacity.
We have also confirmed that the thermal conductivity of transported aggregates
is a crucial parameter which could limit or even suppress the photophoretic
migration and generate several segregation effects
A-STAR: The All-Sky Transient Astrophysics Reporter
The small mission A-STAR (All-Sky Transient Astrophysics Reporter) aims to
locate the X-ray counterparts to ALIGO and other gravitational wave detector
sources, to study the poorly-understood low luminosity gamma-ray bursts, and to
find a wide variety of transient high-energy source types, A-STAR will survey
the entire available sky twice per 24 hours. The payload consists of a coded
mask instrument, Owl, operating in the novel low energy band 4-150 keV, and a
sensitive wide-field focussing soft X-ray instrument, Lobster, working over
0.15-5 keV. A-STAR will trigger on ~100 GRBs/yr, rapidly distributing their
locations.Comment: Accepted for the European Astronomical Society Publications Series:
Proceedings of the Fall 2012 Gamma-Ray Burst Symposium held in Marbella,
Spain, 8-12 Oct 201
Structure prediction based on ab initio simulated annealing for boron nitride
Possible crystalline modifications of chemical compounds at low temperatures
correspond to local minima of the energy landscape. Determining these minima
via simulated annealing is one method for the prediction of crystal structures,
where the number of atoms per unit cell is the only information used. It is
demonstrated that this method can be applied to covalent systems, at the
example of boron nitride, using ab initio energies in all stages of the
optimization, i.e. both during the global search and the subsequent local
optimization. Ten low lying structure candidates are presented, including both
layered structures and 3d-network structures such as the wurtzite and zinc
blende types, as well as a structure corresponding to the beta-BeO type
Quantum transport of two-dimensional Dirac fermions in SrMnBi2
We report two-dimensional quantum transport in SrMnBi single crystals.
The linear energy dispersion leads to the unusual nonsaturated linear
magnetoresistance since all Dirac fermions occupy the lowest Landau level in
the quantum limit. The transverse magnetoresistance exhibits a crossover at a
critical field from semiclassical weak-field dependence to the
high-field linear-field dependence. With increase in the temperature, the
critical field increases and the temperature dependence of
satisfies quadratic behavior which is attributed to the Landau level splitting
of the linear energy dispersion. The effective magnetoresistant mobility
cm/Vs is derived. Angular dependent magnetoresistance
and quantum oscillations suggest dominant two-dimensional (2D) Fermi surfaces.
Our results illustrate the dominant 2D Dirac fermion states in SrMnBi and
imply that bulk crystals with Bi square nets can be used to study low
dimensional electronic transport commonly found in 2D materials like graphene.Comment: 5 papges, 4 figure
A continuous non-linear shadowing model of columnar growth
We propose the first continuous model with long range screening (shadowing)
that described columnar growth in one space dimension, as observed in plasma
sputter deposition. It is based on a new continuous partial derivative equation
with non-linear diffusion and where the shadowing effects apply on all the
different processes.Comment: Fast Track Communicatio
Early SPI/INTEGRAL contraints on the morphology of the 511 keV line emission in the 4th galactic quadrant
We provide first constraints on the morphology of the 511 keV line emission
from the galactic centre region on basis of data taken with the spectrometer
SPI on the INTEGRAL gamma-ray observatory. The data suggest an azimuthally
symmetric galactic bulge component with FWHM of ~9 deg with a 2 sigma
uncertainty range covering 6-18 deg. The 511 keV line flux in the bulge
component amounts to (9.9+4.7-2.1) 10e-4 ph cm-2 s-1. No evidence for a
galactic disk component has been found so far; upper 2 sigma flux limits in the
range (1.4-3.4) 10e-3 ph cm-2 s-1 have been obtained that depend on the assumed
disk morphology. These limits correspond to lower limits on the bulge-to-disk
ratio of 0.3-0.6.Comment: 5 pages, 3 figures, accepted for publication in A&
The sky distribution of 511 keV positron annihilation line emission as measured with INTEGRAL/SPI
The imaging spectrometer SPI on board ESA's INTEGRAL observatory provides us
with an unprecedented view of positron annihilation in our Galaxy. The first
sky maps in the 511 keV annihilation line and in the positronium continuum from
SPI showed a puzzling concentration of annihilation radiation in the Galactic
bulge region. By now, more than twice as many INTEGRAL observations are
available, offering new clues to the origin of Galactic positrons. We present
the current status of our analyses of this augmented data set. We now detect
significant emission from outside the Galactic bulge region. The 511 keV line
is clearly detected from the Galactic disk; in addition, there is a tantalizing
hint at possible halo-like emission. The available data do not yet permit to
discern whether the emission around the bulge region originates from a
halo-like component or from a disk component that is very extended in latitude.Comment: to be published in the proceedings of the 6th INTEGRAL Workshop "The
Obscured Universe" (3-7 July 2006, Moscow
Influence of the Earth on the background and the sensitivity of the GRM and ECLAIRs instruments aboard the Chinese-French mission SVOM
SVOM (Space-based multi-band astronomical Variable Object Monitor) is a
future Chinese-French satellite mission which is dedicated to Gamma-Ray Burst
(GRB) studies. Its anti-solar pointing strategy makes the Earth cross the field
of view of its payload every orbit. In this paper, we present the variations of
the gamma-ray background of the two high energy instruments aboard SVOM, the
Gamma-Ray Monitor (GRM) and ECLAIRs, as a function of the Earth position. We
conclude with an estimate of the Earth influence on their sensitivity and their
GRB detection capability.Comment: 24 pages, 15 figures, accepted for publication in Experimental
Astronom
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